The high pressure feeder, or transfer device, is one of the most basic and important components of the Kamyr continuous pulping system. The high pressure feeder is used to transfer steamed wood chips in a liquid (typically white liquor) from low pressure to the top of the continuous digester, at high pressure. A typical high pressure transfer device comprises a pocketed rotor, a housing, a screen, and pump separably connected to the housing. The pocketed rotor has a plurality of through going pockets, each having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor. The housing encloses the rotor and has an exterior periphery with first through fourth ports disposed around the exterior periphery for registry with the inlets to and outlets from the pockets. The first and third ports are opposite, and the second and fourth ports are opposite, and the first and second ports may be adjacent in the direction of rotation.
In a conventional high pressure feeder screen means are dispose in the third port for screening particles above a predetermined size out of the liquid passing through the third port, and a low pressure pump is connected to the third port to provide the suction for sucking liquid through the third port. A high pressure pump is operatively connected to the second port to provide the flow of liquid under high pressure through the second port. Normally the first port is on the top, and the third port on the bottom, the first port connected to the chips chute, and the fourth port connected to the top of the digester.
While conventional high pressure feeders have functioned very well over the decades they have been in use, there have been relatively few substantive changes to the high pressure feeder over the decades. It has been known that the filling efficiency of the conventional high pressure feeder is approximately 50 to 65% on some chip furnishes, and that is significantly lower than is desired, but to date few significant inroads have been made in substantially increasing that efficiency.
To a large extent, the efficiency of the high pressure feeder is dictated by its ability to obtain the chip chute circulation which carries the chips from the chute into the pockets of the rotor. The chip chute circulation is throttled on the suction side of the chip chute circulation pump the the pressure drop across the screen at the third port. The filling efficiency is also limited by the open area of the slotted screen at the third port. It is necessary that the arcuate bars making up the slotted screen have sufficient thickness and width to ensure that they are strong enough so that they do not easily break. A broken screen bar causes costly down time by requiring a high pressure feeder to be replaced, and high pressure feeders repairs and rebuilds are relatively expensive. The width of the slot is typically a compromise between having the maximum open area for the screen and at the same time avoiding an excess of chips passing through the screen and causing difficulties in the chip chute pump.
According to the present invention a simple arrangement is provided for increasing the open screen area. Despite its simplicity, the invention can result in an increase in filling efficiency of the high pressure feeder of about 10 to 50%, which in turn means an increase in the feeding capacity of a given size of high pressure feeder at practically no cost. The invention also is so simple and straight forward that existing high pressure feeders can easily be retrofit, again with only a small cost if done during normal maintenance. Alternatively or additionally, the larger open screen area will allow the high pressure feeder to be run at a lower angular speed (rpm) while still maintaining the existing feed rate, thereby increasing the life of the feeder, or both slightly longer life and slightly higher feed capacity can be obtained by utilizing the invention. For example, the angular speed can be reduced from about 11 rpm to about 7 to 9 rpm, with no drop off in feeder capacity, or the angular speed could be reduced to say, 10 rpm, with an increase in feeder capacity.
According to one aspect of the present invention, a slotted screen for a high pressure feeder, per se, is provided. The slotted screen comprises: An integral metal body comprising a frame and a plurality of bars. The frame having first and third opposite sides, and second and fourth opposite sides. The plurality of bars comprising a first plurality of arcuate bars extending between the first and third sides of the frame, generally evenly spaced from each other to define generally uniformly dimensioned slots therebetween. And, the plurality of bars further comprising at least one reinforcing bar extending between the second and fourth sides of the frame, substantially transverse to the arcuate bars, and reinforcing the first plurality of bars.
The first plurality of arcuate bars preferably comprise concave and convex mid-point areas, and the at least one reinforcing bar preferably comprises a single reinforcing bar engaging the convex mid-point area of the arcuate first plurality of bars, to reinforce the arcuate bars thereat. The integral metal body may comprise a cast metal body including the arcuate bars and the reinforcing bar. The arcuate bars typically have a width of about 0.29-0.32 inches, and define slots therebetween having a width of about 0.31-0.34 inches. A plurality of threaded apertures are formed in the first and third sides for receiving fastening bolts, and the reinforcing bar typically has a width of about one to two and one-half inches.
According to another aspect of the present invention, a high pressure transfer device for transferring a slurry containing particles, the vast majority of which are above a first size, is provided. The transfer device comprises the following components: A pocketed rotor containing a plurality of through going pockets, the rotor rotatable about an axis, and the pockets having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor. A housing enclosing the rotor, the housing having an exterior periphery and first through fourth ports disposed around the exterior periphery thereof for registry with the inlets to and outlets from the through going pockets, the first port being opposite the third port, and the second port opposite the fourth port and the first and second ports are adjacent in the direction of rotation of the pocket. Means for mounting the rotor in the housing for rotation with respect to the ports about the given axis of rotation, and in a first direction. Screen means disposed in the third port, for screening particles above the first size out of the liquid passing through the third port. And, means for providing a suction source to the third port to suck liquid through the screen means when a pocket is rotated into operative association with the third port. And, wherein the screen means comprises: An integral metal body comprising a frame and a plurality of bars. The frame having first and third opposite sides, and second and fourth opposite sides. The plurality of bars comprising a first plurality of arcuate bars extending between the first and third sides of the frame, substantially evenly spaced from each other to defining substantially uniformly dimensioned slots therebetween. And the plurality of bars further comprising at least one reinforcing bar extending between the second and fourth sides of the frame, and reinforcing the first plurality of bars.
According to another aspect of the present invention, a method of increasing the filling efficiency of a high pressure transfer device is provided. The high pressure transfer device comprises: A pocketed rotor containing a plurality of through going pockets, and rotatable about an axis, the pockets having opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor, and a housing enclosing the rotor and having an exterior periphery and first through fourth ports disposed around the exterior periphery for registry with the inlets to and outlets from the through going pockets, the first port being opposite the third port, and the second port opposite the fourth port, and the first and second ports adjacent in the direction of rotation of the pocket, the rotor rotatable in the housing about an axis of rotation, in a first direction, and an original screen disposed in the third port for screening out particles above a predetermined size from the liquid passing through the third port, the screen having a frame having opposite first and third sides, and opposite the first and third sides, the bars having a first width, and defining between them slots have a second width which is less than the predetermined size of particles to be screened thereby, and the screen devoid of bars extending between the second and fourth sides of the frame. The method of the invention comprises the steps of: (a) Providing a replacement screen having a frame with opposite first and third sides and opposite second and fourth sides, and a plurality of bars extending between the first and third sides, the bars having a third width much less than the first width, and defining between them slots having the second width, so that many more bars and slots are provided between the second and fourth sides than for the original screen, so that the percentage of the open area in the replacement screen is significantly greater than in the original screen, and at least one reinforcing bar extends between the second and fourth sides for reinforcing the bars extending between the first and third sides. And, (b) removing the original screen from the third port of the rotor housing, and replacing it with the replacement screen.
Step (a) is practiced so as to provide the third width size compared to the first width, and the width of the at least one reinforcing bar, so that the percentage of open area of the replacement screen compared to the original screen is about 40-60% greater. For example the third width may be about one-half the first width, yet the slotted screen still has sufficient structural integrity because of the reinforcing bar.
The rotor with the original screen in the third port of the housing is rotated at a first angular speed during use, and has a first capacity, and there is the further step (c) of rotating the rotor with the replacement screen in the third port of the housing at a second angular speed, significantly less than the first angular speed, so that it has a second capacity, substantially the same as, or slightly greater than, the first capacity. Alternatively, the step (c) may be practiced by rotating the rotor with the replacement screen in the third port of the housing at a second angular speed, substantially the same as, or slightly lower than, the first angular speed, so that it has a second capacity significantly greater than the first capacity.